A blog about my cats and feline information in general.
Dedicated to Mikkie, Cato, Charlie and Tina: my feline family members.

Friday, November 23, 2012

Eye see.

Cats’ eyes are
beautiful: big round and deep with fascinating pupils that go from round to
slit. I always wondered how cats see the world and did some reading up on it.
What follows is just a brief summary of what I learned, but it gives a fair
idea of how cats’ eyes work.

Most people
know that cats are nocturnal hunters. Cats don’t just rely on their eyes to
hunt. They have touch-sensitive whiskers, their smell is 14 times better than a
human’s and they can hear sounds 1.6 octaves higher than a human. All this
helps them to hunt at night. They are, however, active during the day as well.
That’s why their eyes have evolved in such a way that they can see very well in
the dark and fairly well in daylight.

A cat’s field
of view is about 200 degrees, which is only a bit larger than that of a human
(180 degrees). Both the pupil and the lens are bigger than the pupil and lens
in the human eye, letting in more light. Their pupils contract from round to
slit, which is a faster process than contracting from big round to small round
as with humans. There is a trade-off for these advantages.When the pupil is in slit mode, (in bright
light) a cat can’t focus as well as when it is round (dim light) and although
the cat’s lens is much bigger than a human’s lens, it can’t change size like a
human’s lens. Therefore a cat’s depth of field is not as good as ours. We can
focus on objects far away and close by. A cat can’t focus on objects close by.
You can compare it to the healthy eyes of an older human. Human eyes loose
their flexibility with age and you end up needing reading glasses for objects
close by.

The lining of
the back of the eye is called the retina. It is where the image focuses and
where the rods and cones are situated. The rods are highly light-sensitive but
not sensitive to colour. They help us see in dim light conditions. The cones
are the ones that are colour-sensitive and also deal with detail detection in
daylight conditions.

The human eye
has an area on the retina called the fovea. This area is very high in cones,
and has virtually no rods. That’s why humans have very good colour vision and
can also detect a lot of finer detail in good light (try seeing how much detail
and colour you can detect in a dark room with only some moonlight coming in
through the window).

The retina in a
cat’s eye contains more rods than cones and doesn’t have an area with only
cones. That’s why cats can see very well in darker conditions, six times better
than humans, but don’t see as much detail and colour in daylight conditions. As
colour is the reflection of light off surfaces, you can’t really see colour in
the dark anyway. Therefore, A nocturnal hunter like a cat doesn’t really need
the ability to see colour that well. Instead of a fovea, cats have an area
behind the retina called the tapetum. This area contains reflective cells that
reflect the light back to the rods and cones, so the slightest bit of light in
the dark is put to good use. That’s why cats’ eyes light up in the headlights
at night: the cells on the tapetum work as mirrors.

Cats can give a
really intense stare because, unlike humans, cats don’t blink to moisten their
eyes. Cats do blink slowly as a form of communication. Some people call it cat
kisses. Slow blinking makes cats feel at ease and is a show of affection. Try
it out some time. If the cat slow blinks back at you, it means that you are communicating
successfully, in cat language, with your cat.

Have you ever
looked at a sleepy cat that has just opened the eyes slightly and seen a
whitish membrane partly over the eyes? Well, you were looking at their third
eyelid, called the nictitating membrane. This is a translucent membrane that
moves from the side. It protects the eye and keeps it moist while maintaining
visibility. This membrane is sometimes slightly visible when a cat is sleepy
and content. It is also visible when a cat is sick. If the membrane stays in
view while the cat is awake, you should contact your vet and get it checked
out.

Some people
still believe that cats are completely colour-blind. This is not true. Cats can
see colours, but not all colours. They can distinguish between blues and greens
but not between shades of red.

So to recap: a
bigger lens and pupil, combined with a higher density in rods and the
reflective cells on the tapetum, allow a cat to see 6 times better in the dark
than humans. Cats’ eyes are highly light sensitive but, as a trade-off, lack
the ability to detect a lot of detail and colour sensitivity in daylight. A cat
can see certain colours (greens and blues) and is therefore only partly colour-blind.
A cat can’t focus on objects close by because of non-flexible lenses. Cats
don’t blink to keep their eyes moist. They slow blink to communicate. They have
a third eyelid called the nictitating membrane that protects their eyes and
keeps it moist.

Right, I will
include a few pictures and then I’m done. I hope you liked this little
write-up. :)